1. Field of the Invention
The present invention relates to a light source which blinks at high rates. Particularly, the invention relates to a light source which can blink at a rate high enough to be utilized for optical communication.
2. Description of the Related Art
There exist a large number of situations in which an optical communication system is desired to be built in a relatively short distance and to transmit a large amount of information. Investigations are now under progress for building an optical communication system in such situations using plastic optical fibers (hereinafter called the “POF”) at a low cost.
In optical communication systems which utilize the POF or the like, an attempt has been made to utilize light emitting diodes made of group III-V compounds. For example, researches have been made for utilizing a green or a blue light emitting diode of a GaN (gallium nitride) based compound, or a red light emitting diode of an AlGaInP-based compound for a light source of an optical communication system which utilizes the POF. With the use of light emitting diodes of group III-V compounds, it is possible to produce light at wavelengths with small transmission losses of POF.
In an optical communication system, a light emitting diode is blinked by applying a pulse voltage to the light emitting diode. For increasing the transmission rate, it is important to reduce a fall time of the light emitting diode. The fall time, herein referred to, means a time until the light output of the light emitting diode is reduced when the pulse voltage applied to the light emitting diode is switched from a high level to a low level. Specifically, when the light output of the light emitting diode when applied with a voltage at high level is designated by P1, and the light output of the light emitting diode when applied with a voltage at low level is designated by P2, the fall time is a time required until the light output calculated by P2+(P1−P2)×0.9 changes to the light output calculated by P2+(P1−P2)×0.1. The blinking, herein referred to, means a reduction of the light output when the light emitting diode is applied with a voltage at low level below the light output when applied with a voltage at high level, and is not limited to a reduction of the light output to zero.
A technique for reducing the fall time of the light emitting diode is disclosed in JP-A-2003-163377. The technique of JP-A-2003-163377 utilizes a pulse voltage which presents a small difference between a voltage at low level and a voltage at high level, as shown in FIG. 5A. This example utilizes a pulse which has the voltage at high level set at 3.1 V, and the voltage at low level set at 2.8 V.
A solid line 42 shown in FIG. 5B indicates a change over time of a light output when the pulse voltage shown in FIG. 5A is applied to a light emitting diode. A broken line 44 shown in the figure indicates a change over time of the light output when the voltage at low level is set at 0 V (ground potential)
When the voltage at low level is set at the ground potential, the fall time is t1, whereas when the voltage at low level is set higher than the ground potential, the fall time is reduced to t2. The technique of JP-A-2003-163377 successfully reduces the fall time by continuously applying a current to the light emitting diode even during the low level to pull up the light output during the low level.
The technique of JP-A-2003-163377 is an extremely useful technique in regard to a reduction in the fall time. As long as prevailing light emitting diodes made of group III-V compounds are utilized, it can be said that this is the unique technique for reducing the fall time.
However, the technique of JP-A-2003-163377 has a problem in that a light output amplitude (difference between the light output P1 when the light emitting diode is applied with the voltage at high level and the light output P2 when applied with the voltage at low level) becomes smaller. As the light output amplitude is smaller, a special technique is needed in a receiver circuit which discriminates whether the light output is at high level or low level. A problem still remains in that a transmission distance is limited even if the special receiver circuit is utilized. The technique of JP-A-2003-163377 can be said to be an actual unique technique which can reduce the fall time by bringing the voltage at low Level to the voltage at high level, as long as the prevailing light emitting diodes made of group III-V compounds are utilized, so that the disadvantage of the reduced light output amplitude is overcome with hard efforts.